There has been known a technique in which eight receiving antennas are arranged in a horizontal direction with the first and the eighth receiving antennas being shifted in an upward direction from the other receiving antennas, and the azimuth in the vertical direction of a target is obtained from a first oblique direction formed by the first receiving antenna and the second receiving antenna, and from a second oblique direction formed by the seventh receiving antenna and the eighth receiving antenna (for example, refer to a first patent document).
In this technique, DBF (digital beam forming) processing is carried out on signals obtained by the first through eighth receiving antennas, so that a distance, a relative speed and a horizontal angle of the target are detected. After that, the azimuth of the target with respect to the first oblique direction and the azimuth of the target with respect to the second oblique direction are respectively detected by the use of a phase monopulse system, and the azimuth in the vertical direction of the target is obtained from two detection results.
However, the detection accuracy and the detectable azimuth of the target change in accordance with the intervals and angles at which the receiving antennas are arranged. Here, the accuracy and the range of detection required vary, for example, between targets at a long distance and at a short distance. In addition, the performance required also varies with traveling conditions, surrounding environments, etc. In cases where the combination of receiving antennas to be used is set in advance, as in the conventional technique, it becomes impossible to exhibit the required performance, depending on conditions or situations.
For example, if the detection accuracy of the azimuth in the vertical direction is low, an iron plate or steel sheet laid on a road will be detected as an obstacle, thus giving rise to a fear that an unnecessary warning or a brake operation may be made, in spite of the fact that a vehicle is able to pass over the iron plate. However, it becomes impossible to detect a wide range of area by means of a radar which is specialized only in the performance or ability of detecting such an iron plate.